The present invention relates to a process for the protection of flexible bi-constituent silicone membranes, more particularly flexible self-demoulding silicone membranes, particularly in the case of use in moulding by resin transfer.
A bi-constituent silicone has the following general formula when it is obtained by poly addition: ##STR1##
Resin transfer moulding consists in preparing forms having confronting surfaces generating a volume of the shape of the piece to be made, placing on the walls of this volume a flexible silicone membrane to permit demoulding the finished piece, and injecting into the space delimited by said membrane, resin and its polymerization catalyst. The mould walls can be heated so as better to control the polymerization reaction.
This process is widely used in the automotive industry and in the field of sports and leisure. This process permits obtaining better technical performances, reducing the production cycles.
Initially used only for small-size pieces, this process tends to be more widespread and to find applications including the production of structures of large size, as in the aeronautical field.
Moreover, the injected resins are of the phenolic type and have particularly interesting advantages in this aeronautical field, because they are self-extinguishing and the fumes given off, of small volume, are not harmful.
Each resin is sold with its catalyst and there is more particularly known one among others which contains paratoluene sulfonic acid.
This catalyst is well known for this property as to certain resins but it has on the other hand drawbacks as to health, because it is an irritating acid but above all it can become noxious at the temperatures at which it is used, in addition to being costly.
The flexible self-demoulding silicone membrane itself has numerous qualities which are essential for the good practice of the process. There is no machining, it is easy to demould because one need only pull on the membrane to peel it relative to the piece, and the surface condition of the obtained piece is excellent.
On the other hand, this membrane has an important drawback: in contact with the resin, and more particularly the phenol groups, the membrane corrodes. This corrosion has the effect of degrading the condition of the surface and this phenomenon becomes progressively more marked upon successive uses.
This premature aging causes the membrane to lose all its demoulding qualities and accordingly leads to a decrease of the quality of the surface condition of the piece. This fact requires the user to replace the membrane frequently, which replacement decreases the profitability of the process of resin transfer moulding.